Behavior of colloidal gels made of thermoresponsive anisotropic nanoparticles

Amongst colloidal gels, those designed by the assembly of anisotropic colloidal particles tend to form fibrillar gels and are attracting interest as artificial cell growth environments since they have a structure reminiscent of biological extracellular matrices. Their properties can be tuned by controlling the size, shape, and rigidity of the nanoparticles used during their formation. Herein, the relationship between the physical and mechanical properties of the nanocolloidal building blocks and the properties of the resulting gels is investigated. Thermoresponsive particles with different aspect ratios and controlled rigidity were prepared, and the gelation and the properties of the resulting gels were studied. The results show how the aspect ratio and rigidity of polymer colloids tune the properties of the gels. An increase in the aspect ratio of the nanocolloid used led to a sol–gel transition observed at lower particle concentration, but an increase in the rigidity of the nanocolloids delayed the sol–gel transition to higher concentration. However, at a constant concentration, increases in the anisotropy produced gels with higher modulus and lower yield strain. Similarly, an increase in rigidity of the colloids increased the modulus and reduced the yield strain of the resulting gels.


H NMR Spectroscopy
All NMR spectra were recorded on a Bruker Avance 300 MHz.

Dynamic Light Scattering (DLS)
Dynamic light scattering measurements were performed using a Malvern Zetasizer Nano S90 using a He−Ne laser (633 nm) and analysis angle of 90°. The hydrodynamic diameter of the polymer structures was measured in water. All samples were filtered with 0.45 μm filter before measurements.

Transmission Electron Microscopy (TEM)
The nanostructures were analyzed using a JEOL-1400 transmission electron microscope at an accelerating voltage of 120 kV. Typically, TEM grid preparation was as follows: the samples were diluted with Milli-Q water to approximately 0.05 wt %. The TEM grid was dipped in the diluted solution, the excess solution was blotted with a filter paper, and the grid was then dried at room temperature. To stain the deposited nanoparticles, a 0.4% w/w aqueous solution of uranyl acetate was placed via micropipette on the grid for 20 s and then carefully blotted to remove excess stain.

Image analysis
The TEM images were analyzed using MATLAB. First, the TEM micrographs were converted into binary images. The build-in function regionprops was used to identify the different objects present in the image. Then, the image was skeletonized (bwmorph) to identify branched or overlapping worms. The end-to-end distance was measured along each skeleton (bwdistgeodesic).
The cosinus of the angle between two tangents of the worms was calculated as the dot product of the tangent vectors divided by the product of their lengths. The values reported are the average of all the particles measured (N>75).

Rheological Measurements
Rheological measurements were carried out on Bohlin Gemini rheometer equipped either with coaxial cylinder geometry with a measuring bob radius of 25 mm, a measuring cup radius of 27.5 mm and 13.0 mL sample volume or a cone and plate geometry (1° cone angle and 40 mm diameter with a truncation gap of 500 μm). The viscosity of the suspension was measured in shear-sweep experiment from 0.01 to 1 Hz and used to extrapolate the zero-shear viscosity of each sample. For the dynamical measurements, about 5 mL of gel was loaded onto the plate, and the cone was lowered to minimize the truncation gap, and the excess of gel was removed. Before experiments, samples were equilibrated in the geometry for 10 min. Oscillatory strain sweep measurements were run from 0.01 to 500% deformation at a fixed frequency of 1 rad/s. Oscillatory temperature sweep measurements were performed at different temperatures at a frequency of 1 rad/s, with a constant strain of 1%, which is within the linear viscoelastic regime of the hydrogels. Temperature for the frequency step was maintained from 25 to 45 °C for each frequency sweep.
The monomer conversion was analyzed by 1 H NMR spectroscopy. PDMA was purified by precipitation into cold hexane three times and dried under vacuum to yield a yellow product. The PDMA macro-CTAs were characterized by 1 H NMR spectroscopy and GPC.

Synthesis of micelles (PDMA31-b-PBzMAx) via Dispersion Polymerization in Ethanol
A typical RAFT dispersion polymerization was conducted at a concentration 15% w/w total solid fraction. First, PDMA31-CTA (144 mg, 0.028 mmol), BzMA (1.5 g, 8.4 mmol), and AIBN (0.90 mg, 0.006 mmol) were dissolved in ethanol (9.32 g), the reaction mixture was degassed with argon in an ice/water bath for 20 min, and then placed in a preheated oil bath at 70 °C for 24 h. The length of the PBzMA block was varied by using different amount of BzMA (0.10 g to 1.5 g).
According to 1 H NMR spectroscopy analysis, the final conversion of BzMA was above 99%. The resulting polymers were analyzed by 1 H NMR spectroscopy and GPC.
of the PBzMA block was systematically varied, which allowed to tune the morphology of the micelles.

Synthesis of Crosslinkable micelles (PDMA31-b-PBxMA35-co-AAEM5)
The synthesis of crosslinkable block copolymer was performed similarly to the synthesis of In order to control the density of crosslinking, the DP of AAEM of PDMA31-b-PBzMAx-co-AAEMy was varied at 5 and 10, respectively but keeping x+y=40. This was realized by tuning the amount of AAEM and BzMA in the monomer mixture.